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Beyond Plate Tectonics: Looking at Plate Deformation with Space Geodesy

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Beyond Plate Tectonics: Looking at Plate Deformation with Space Geodesy Beyond Plate Tectonics: Looking at Plate Deformation with Space Geodesy THOMAS H. JORDAN Department of Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology, Cambridge, MA 02139 J. BERNARD MINSTER Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography La Jolla, CA 92093 ABSTRACT. We address the requirements that must be met by space-geodetic systems to place useful, new constraints on horizontal secular motions associated with the geological deformation of the earth's surface. Plate motions with characteristic speeds of about 50 mm/yr give rise to displacements that are easily observed by space geodesy. However, in order to improve the existing plate-motion models, the tangential components of relative velocities on interplate baselines must be resolved to an accuracy of < 3 mm/yr. Because motions considered small from a geodetic point of view have rather dramatic geological effects, especially when taken up as compression or extension of continental crust, detecting plate deformation by space-geodetic methods at a level that is geologically unresolvable places rather stringent requirements on the precision of the measurement systems: the tangential components on intraplate baselines must be observed with an accuracy of < 1 mm/yr. Among the measurements of horizontal secular motions that can be made by space geodesy, those pertaining to the rates within the broad zones of deformation characterizing the active continental plate boundaries are the most difficult to obtain by conventional ground-based geodetic and geological techniques. Measuring the velocities between crustal blocks to ± 5 mm/yr on 100-km to 1000-km length scales can yield geologically significant constraints on the integrated deformation rates across continental plate-boundary zones such as the western United States. However, baseline measurements in geologically complicated zones of deformation are useful only to the extent that the endpoints can be fixed in a local kinematical frame that includes major crustal blocks. For this purpose, the establishment of local geodetic networks around major VLBI and SLR sites in active areas should receive high priority. 1. INTRODUCTION Deformation of the earth's lithosphère covers a broad spectrum of temporal and spatial scales, from seconds to aeons and from mineral grains to planetary dimensions. Table 1 categorizes a subset of lithospheric motions that cause geologically and geophysically significant deformations. It is convenient to discriminate secular motions persisting on 341 M. J. Reid and J. M. Moran (eds.), The Impact of VLBI on Astrophysics and Geophysics, 341-350. ©1988 by the IAU. Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 02 Oct 2021 at 17:17:47, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900134941 342 geological time scales of thousands to millions of years from transients associated with, for example, seismic and volcanic events. Practical research is more concerned with the transients, because they tend to disturb human activities. Secular motions also warrant vigorous study, however, since they provide the kinematical framework for describing transients and understanding their driving mechanisms. TABLE 1. Types of motions observed at the earth's surface. SECULAR TRANSIENT Plate motions (Pre,co,post)-seismic HORIZONTAL Boundary-zone tectonics Fault creep Intraplate deformation Stress redistribution Tectonic motions (Pre,co,post)-seismic Thermal subsidence Magma inflation VERTICAL Diapirism Tidal loading Crustal loading Post-glacial rebound Cratonic epeirogeny The most significant long-term deformations are those related to plate tectonics. Although local tectonic movements near plate boundaries display large vertical components and time-dependent behavior, the net motions between the stable interiors of large blocks are forced by viscous damping and gravity to be nearly steady and horizontal. The characteristic tangential velocity of the plate system is about 50 mm/yr, which gives rise to displacements easily measured by geodetic methods. Horizontal secular motions have been observed both by ground-based networks [e.g., Savage, 1983] and by space-geodetic systems [e.g., Christodoulidis etal., 1985; Herring etal., 1986]. Though their application to geodesy is relatively new, space-based techniques — Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Positioning System (GPS) — have already revolutionized the science of terrestrial distance measurement. They are contributing new information about active tectonics, particularly on the planetary scales previously inaccessible to ground-based surveys (Figure 1). SLR VLBI — GPS ? ? — Ground-based methods • I I I I I I I I -1 0 1 2 1(T3 10,- 2 ίο ίο ίο io 103 104 Baseline Length (km) Figure 1. Spatial scales sampled by various geodetic methods. Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 02 Oct 2021 at 17:17:47, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900134941 343 In this paper, we examine the space-geodetic observations needed to solve geologically interesting problems. Specifically, we are concerned with the measurement accuracy required to place useful, new constraints on models of global plate velocities and related horizontal secular motions. 2. PLATE MOTIONS In the ocean basins, most of the deformation related to horizontal secular motion occurs in well-defined, narrow zones that are the boundaries of a dozen or so large lithospheric plates. The current plate velocities are constrained by three basic types of data collected along these submerged boundaries: (1) spreading rates on mid-ocean ridges from magnetic anomalies, and directions of relative motion from (2) transform-fault azimuths and (3) earthquake slip vectors. The first self-consistent global models were synthesized soon after the formulation of plate tectonics [Le Pic hon, 1968], and significant refinements were made throughout the next decade [Chase, 1972; Minster et al., 1974]. Third-generation models were published by 1978 [Chase, 1978; Minster and Jordan, 1978] and are still in use. The velocity field for the RM2 model of Minster and Jordan [1978] is depicted in Figure 2. VELOCITY SCALE: • - IOcm/Yr Figure 2. The plate geometry and velocity field for the eleven-plate RM2 model of Minster and Jordan [1978] in the AMl-2 (hotspot) reference frame (map courtesy of A. Forte and W. Peltier). Since the reference frame is arbitrary, the angular velocity vectors describing the instantaneous relative motions among M rigid plates are specified by 3(M -1) independent components. The 30 numbers representing the eleven-plate RM2 model were derived by least-squares inversion of a carefully selected, globally distributed data set comprising 110 spreading rates, 78 fault azimuths, and 142 slip vectors. The magnetic anomalies employed in this analysis average the rates over the last 2-3 million years, about the shortest time span for which good spreading rates can be obtained on a global basis. Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 02 Oct 2021 at 17:17:47, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900134941 344 Although the interval sampled by RM2 is hardly "instantaneous" from a geodetic point of view, it is geologically brief, and the small plate displacements that take place during it are well described by infinitesimal (as opposed to finite) rotations. TABLE 2. Comparison of extension rates, in mm/yr, for two transatlantic baselines. Baseline VLBI* RM2t Haystack-Onsala 17 ±2 17 ±3 Ft.Davis-Onsala 20±7 16±3 * CDP/IRIS data from Herring et al. [1986]. 1" Calculated for NOAM-EURA plate pair. It will probably be some time before the global plate-tectonic models can be significantly improved by space geodesy. Because the geological data sets are large and the inverse problem is strongly overdetermined, the formal uncertainties in the angular velocity components are already fairly small. RM2 predicts interplate baseline rates-of-change with standard errors that are typically a few millimeters per year (mm/yr) [Minster and Jordan, 1983]. Consider, for example, the two transatlantic baselines in Table 2, which connect the VLBI antenna at Onsala, Sweden, with antennas in Massachusetts and Texas. Positions between these sites have been measured at relatively frequent intervals under the auspices of the International Radio Interferometric Surveying (IRIS) project and NASA's Crustal Dynamics Project (CDP). From an analysis of the 1980-1984 data, Herring et al. [1986] have found rates of motion between the Eurasian and North America plates that are in good agreement with the RM2 estimates. This is comforting, both as a check on the techniques and as corroboration of the geophysical expectation that the instantaneous velocities between points in stable plate interiors are dominated by secular plate motions. However, if the 3-mm/yr prediction errors of the plate model can be believed, high-quality VLBI data sets over a number of such baselines will be needed to really refine the geological estimates. There are other plate pairs for which the currently available global models are not as good, and some improvement could be made from space-geodetic data. In Figure 2, for example,
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